This invention relates to high-intensity-discharge (HID) lamps and, more particularly, to improved electron emissive material for the electrodes of such lamps.
In U.S. Pat. No. 3,708,710 dated Jan. 2, 1973 is disclosed a high-intensity-discharge sodium-mercury vapor lamp which utilizes dibarium calcium tungstate as electron emissive material. Such material has been used in so-called dispenser cathodes and U.S. Pat. No. 3,434,812 dated Mar. 25, 1969 discloses the use of dibarium calcium tungstate or dibarium strontium tungstate as an emissive material in a dispenser cathode.
Dibarium calcium molybdate is known for use as a getter layer material in conjunction with an incandescent lamp, as disclosed in U.S. Pat. No. 3,266,861, dated Aug. 16, 1966. In addition, high-pressure mercury-vapor lamps and sodium-mercury vapor lamps have in the past utilized as electron emissive material a mixture of several oxide phases comprising thorium dioxide, barium thorate, dibarium calcium tungstate and barium oxide. This mixture of oxide phases is quite sensitive to the atmospheric contaminants with the result that even a brief exposure to the air can result in a relatively large pickup of water and carbon dioxide by the emission mixture, which contaminants are rather difficult to remove. In such a mixture, the thorium dioxide serves as a matrix for the more active oxide emitters, such as barium oxide, dibarium calcium tungstate and barium thorate.
In U.S. Pat. No. 4,052,634 dated Oct. 4, 1977 to DeKok is disclosed an HID lamp having an electrode consisting of a support of a high-melting metal provided with an electron emissive material. The emissive material consists mainly of one or more oxide compounds containing (a) at least one of the rare earth metal oxides, (b) alkaline earth metal oxide in a quantity of 0.66 to 4 mole per mole of rare earth metal oxide and (c) at least one of the oxides of tungsten and molybdenum in a quantity of 0.25 to 0.40 mole per mole of alkaline earth metal oxide, with the alkaline earth metal oxide consisting of at least 25 mole % of barium oxide.
The compounds Ba.sub.3 CaNb.sub.2 O.sub.9 and Ba.sub.3 CaTa.sub.2 O.sub.9 are known as perovskite-type compounds, as disclosed in "Structure, Properties and Preparation of Perovskite-Type Compounds:" by Galasso, Pergamon Press (1969), see page 25 thereof.